28 July 2010

Michael Oppenheimer Responds

[Michael Oppenheimer of Princeton University has graciously offered this post in response to several comments made earlier this week on his recent PNAS paper on climate change and Mexico-US immigration. His contribution is much appreciated. -RP]

Our article in Proceedings of the National Academy of Sciences, “Linkages among climate change, crop yields and Mexico–US cross-border migration" by Shuaizhang Feng, Alan B. Krueger, and Michael Oppenheimer“, has drawn criticism in this blog. Before we respond in detail, let us point out that the problem of climate-driven migration has resisted quantitative analysis for decades. To contribute to filling this gap, we proposed a line of modeling drawing on empirical evidence about responses to past climate variations, which may be informative about responses to future climate trends. If others have better ideas for estimating how climate change affects migration, they should publish them.

Richard Tol argues that our paper “confuses decadal weather variability with climate change”. We were not confused and I doubt anyone who read the entire article would be confused either. Using the response to historical short term variability to estimate the response to a long term trend is a common first approximation in climate impact analysis when quantitative process-based information is lacking, such as for the processes underlying the immigration response. We state in the paper,

“Such a method implicitly assumes that the response to changes in the climate variables is linear and symmetric, and that migration responses to 5-y changes in climate conditions can be applied to longer run trends. We acknowledge that, in actuality, the response to a trend may differ from the response to periodic variability...”

... particularly because climatic variability during the study period has not been of a magnitude or duration comparable to what is projected in the future under various climate scenarios.

Richard further argues that the study

“fails to control for other determinants of migration that may well be correlated with weather during the sample.”

First, in terms of estimation, our approach (fixed effects IV) controls all time-unvarying state effects through state dummies, and addresses (controls for) other determinants of migration that are NOT correlated with climate through using climate as the instrument. If some of those determinants are correlated with changes in climate (other than fortuitously), then the estimated semi-elasticity would be affected, but we would be just attributing some of the impacts of climate on emigration through other channels to the crop yield channel (as the two cannot be separated anyway given the data).

Second, in term of prediction, we emphasize from the beginning that our study is a “ceteris paribus” one. In the language of economics, it is a “partial equilibrium”, not a “general equilibrium” study. The objective is to examine the “marginal” effect of climate change on migration, holding everything else constant, not to forecast the “total” number of emigrants in the future.

Finally, Tol states,

“they extrapolate beyond belief... their largest yield change is -48% between now and 2080. If technological progress would bring about a 1% yield increase per year, then the two effects cancel each other out. 1% may be too low, -48% is probably too high.”

We are careful to acknowledge the impact of other factors including technological progress in the paper:

“estimate of the elasticity of migration is conditional on many factors specific to Mexico for the period under study, such as the macroeconomic situation compared with that of the United States, the population share of youths (who are more likely to migrate), farming practices, the relative importance of the agricultural sector, and agricultural policies including responses to droughts and other climatic events that adversely affect crop yields.”

However, note that we are estimating the “marginal” effect of climate change. If technological change produces general increases in productivity and partially offsets effects of climate change, then predictions of “total” migrants would change, but the “marginal” effect of climate change would not.

With respect to the observation of Harrywr2 that at the upper end, our projections exceed the current size of the agricultural labor force: First, people not employed in the agricultural sector would also be affected by declines in crop yield, due to a well known and substantial “ripple effect”, and these would also be represented in our empirically determined semi-elasticity. Furthermore, the projection of between 1.4 to 6.7 million Mexicans migrating to the U.S. due to climate change applies to a 70-year time period. Thus, more than one generation would be affected.

Second, the projection is based on the assumption that Mexican adult population would stay at today’s level of 70 million. The projected numbers are significant but not necessarily “alarming” if they are put into the right context. The recent annual Mexican emigration flow to US is about 500,000. If in the future this is cut by half on average due to factors such as declines in fertility, to 250,000 emigrants, the total number emigrating due to all other causes would be 17.5 million in the 70 years.